The dewatering of fibrous materials is commonplace in industry. For example, in the art of paper making, fibrous pulp must be dewatered before it is processed into its final form. In many instances the dewatered pulp is shipped after dewatering and stored until such time as it is used. The degree of moisture retention in the dewatered pulp affects the utility as well as the economics of the process. In storage, the pulp is subject to bacterial degradation. The rate of such degradation increases with increased moisture content. And, an increase in moisture content increases the cost of shipping the dewatered material.
For these reasons and others, it has been the objective of prior art inventors to provide dewatering methods and apparatus that will reduce the water content to the greatest extent possible. Whereas, commonly used methods such as screw presses, revolving discs, V-presses or roll type presses can dewater to a presscake with 60 to 70 percent moisture, it is highly desirable to obtain a presscake with 30 to 40 percent moisture.
The characteristics of moisture release from fibrous materials are not like granular material because of the matting action of the fibers, the resilience of the mat formed, the retention of moisture in the soft porous structure of the fibers themselves, and the trapping of fines and material other than fibers in the mat.
While I do not necessarily mean to be bound by any theory as to why the herein described method and apparatus are successful, it is believed that the following theories explain such success.
I believe the general factors which affect the degree of moisture released are: (1) pressure must be applied sufficient to reduce volume of the matted material to the necessary degree; (2) pressure must be applied in a manner to avoid or reduce thrust on screens; (3) the travel path of moisture must be reduced by thin cross-section of material and close spacing of openings in screen; and (4) sufficient time for moisture travel must result from press action.
In U.S. Pat. No. 3,863,559, incorporated by reference herein, there is disclosed a method for dewatering a presscake to 30 to 40 percent moisture. The method in terms of certain process conditions including:
(1) pressure applied parallel to the screen surface; PA0 (2) a compression rate that begins at about 8 feet per minute and which is reduced to 2 feet per minute toward the end of the cycle; PA0 (3) 300-3,000 psi pressure applied to the presscake and a uniform thin cross-sectional area, about 1/2 to 3/4 inch.
The patent also describes an apparatus for practicing the method.
The press as hereinafter described is adapted to practice the method of U.S. Pat. No. 3,863,559, although other methods may also be practiced with it, and is designed to be an improvement over the apparatus of the U.S. Pat. No. 3,863,559.